The current accepted thinking is that global warming is due to green-house gas emissions, such as carbon dioxide (CO2) and methane (CH4). About a quarter of global carbon dioxide emissions currently come from mobile sources. This fraction could grow rapidly with the projected surge in car ownership in developing countries. Since nitrogen comprises almost 80% of the gas in the earth's atmosphere, its presence in the combustion chamber of internal combustion engines cumulatively results in a substantial volume of oxidation products, referred to generally as NOx pollutants. Although the catalytic converter is able to convert many of these undesirable compounds, it has been recognized that the reduction or elimination of nitrogen from the air/fuel mixture would be highly desirable. Air pollution management from mobile sources has many challenges, such as space and weight limitations, the economy of scale working against this application and fast dynamic operation of the mobile sources.
An oxy-combustion process for use in stationary power generation plants or in internal combustion engines, is proposed in U.S. Pat. No. 6,170,264 which process includes an air separation step for separating nitrogen from the air prior to the use of the air to combust a hydrocarbon fuel so that nitrogen oxide and other pollutants are reduced or eliminated as by-products of combustion. A further reduction of harmful pollutants such as sulfur, sulfides and various nitrogen oxides (NOx) is proposed by using highly refined fuel such as hydrogen, methane, propane, purified natural gas, and light alcohols such ethanol and methanol.
It is specifically proposed in the '264 patent that an air separation plant use a membrane-based air separation system to separate the air into its component parts by passing an air feedstream under pressure over a membrane. The pressure gradient across the membrane causes the most permeable components to pass through the membrane more rapidly than other components, thereby producing a product stream that is enriched in the permeate component, while the original feedstream is depleted in that component. Many membranes can operate at ambient temperatures. Several types of membranes and their characteristics are described. Cellulose acetate membranes are said to exhibit good separation factors for oxygen and nitrogen, but have relatively low flux rates. Film composite membranes placed over a microporous polysulfone substrate exhibit lower separation factors than cellulose acetate, but have higher flux rates at the same pressure differential. Providing multiple membranes in series can increase the oxygen concentration in the product stream.
Electroceramic membranes, which are ionic solid solutions that permit movement of ions, require relatively higher temperatures of about 800° F. to mobilize the oxide ion when separating oxygen from the air feedstream. In the example of FIG. 12 in the '264 patent, the combustion exhaust gas, referred to as the “working fluid” is routed to a heat exchanger which in turn heats the electroceramic membrane to the desired operating temperature of 800° F. The patent identifies yttria stabilized zirconia as a possible material for the electroceramic membrane.
The '264 patent also contemplates that a membrane could be used to pass nitrogen and thereby reduce the nitrogen content of the remaining air feedstream. In this mode of operation, the nitrogen enriched stream will be on the outlet side of the membrane and the oxygen-enriched stream will be the retentate. Since the rate of diffusion through the membrane is determined by ion mobility, which in turn is a characteristic of a particular material and is dependent on the size, charge and geometry of the cations in the lattice, the geometry and location of the electroceramic membrane(s) will be determined by its mode of operation.
U.S. Pat. No. 5,051,113 discloses an air intake system for mobile engines that utilizes a selectively permeable membrane to effect oxygen enrichment of the air entering the engine intake in order to improve engine efficiency. The disclosure of U.S. Pat. No. 5,051,113 is incorporated herein by reference. The system utilizes a perfluorodioxole membrane that has an oxygen/nitrogen selectivity of at least 1.4:1 and provides from about a 10% to a maximum of 66% increase in O2 under optimum conditions, and otherwise from about 10% to 30% increase of O2 in the intake gas. Unlike oxy-combustion, as a result of the limitation of the membrane oxygen/nitrogen selectivity, the '113 patent's oxygen enrichment process does not eliminate the majority of the NOx pollutants.
A major problem associated with use of the oxy-combustion process in motor vehicles powered by internal combustion engines is how to minimize the additional weight and space required by the air separation components that are disclosed as necessary for the practice of the process by the prior art, e.g., the '264 patent. For example, size and weight of the additional apparatus required for the step of cycling of exhaust gases through a heat exchanger in order to achieve the 800° F. operating temperature for the electroceramic membrane will be seen by the automotive designer concerned with miles per gallon ratings and vehicle weight to be a significant disadvantage of that process.
In fact, most of the problems faced in reducing pollution emissions from motor vehicles are not present in addressing the reduction of the same pollutants from fixed electrical power generation plants, since floor space and/or overhead space is not limited. Electrical power and other utilities are also readily available in power generation plants to run auxiliary equipment, such as the compressors that are used in the air separation step.
Thus, one problem to be solved is how to achieve the known advantages of oxy-combustion in an ICE used to power a motor vehicle, while minimizing adverse effects on the overall efficient operation of the motor vehicle that are associated with the increase in weight of additional components and the power requirements associated with the air separation step.